The invention relates to a method for producing a multi-dimensionally profiled preform from a pre-impregnated or binder-crosslinked, multidirectional fiber composite material. In such a method, a top tool component and a bottom tool component are used. In the closed position these two tool components define between them a shaping gap, which is profiled in conformity with the preform, wherein in the course of closing the tool components, the fiber composite material is re-tightened by way of a tensioning frame, which envelops the tool components, while simultaneously retaining the tensioning force.
During the production of multidimensional preforms that are made of a fiber composite material that is flexible, yet also stable to tension in the fiber direction, the situation may arise that during the forming process in the forming tool the material deforms between the zones of higher deformation rather than in the zones of less deformation. In this case the material deformation can no longer be compensated for by stretching the material and, as result, becomes apparent in the formation of folds or any other kind of defect in the preform.
In contrast, the object of the present invention is to design a method and a forming tool of the type described at the outset in such a way that the resulting preforms show a significant improvement in quality during production.
The invention achieves this and other objects by a method, and a forming tool used therein, for producing a multi-dimensionally profiled preform from a pre-impregnated or binder-crosslinked, multidirectional fiber composite material by way of a top tool component and a bottom tool component. In the closed position these two tool components define between them a shaping gap, which is profiled in conformity with the preform, wherein in the course of closing the tool components, the fiber composite material is re-tightened by way of a tensioning frame, which envelops the tool components, while simultaneously retaining the tensioning force. In order to compensate for the excess material in regions where the shaping gap has a weaker profile, the fiber composite material is increasingly deformed in the form of a bead in the edge trimming zone of the preform between the shaping gap and the tensioning frame in synchronism with the closing movement of the tool components at the start of the material forming process.
Alternatively or additionally, the forming tool includes a tensioning frame, which envelops the tool components. In the course of closing the tool components, the tensioning frame fixes the fiber composite material in such a way that allows the fiber composite material to be subsequently fed in such that it is frictionally engaged. The tensioning surface of the tensioning frame is vertically adjusted relative to an inlet surface of one of the tool components in such a way that a drawing edge, which is effective in the forming phase and which is provided for the fiber composite material, which subsequently flows into the shaping gap, is formed on the tensioning surface.
According to a first embodiment of the invention, the differences in the profile length of the shaping gap during the forming phase of the fiber composite material is largely compensated for by the fact that in the region of the subsequent edge trimming process of the preform between the tensioning frame and the shaping gap, the fiber composite material is gathered increasingly in the form of a bead at the points where the profile length is less, with the result that the tensile stress of the fiber composite material in the shaping gap and the material after-draft along the tensioning frame become highly uniform during the forming process. And, even in the event that the geometry of the shaped part is complex, it is possible to achieve a significant improvement in the molding on of the material at the shaping gap contour without the disturbing material defects and, in particular, the fold formations by a method that is easy to carry out in the manufacturing process.
Preferably, the drawing-through length and, thus, the material consumption, in the drawing-through gap, is not constant, but rather changes transversely to the draft direction, as a function of the local differences in the profile lengths. This arrangement allows once again the adaptation of the material to the geometry of the shaping gap to be significantly improved even more.
In order to ensure in a simple way that the material consumption in the drawing-through gap is covered by the excess material on the part of the shaping gap of the tool components and not by the fiber composite material that is re-tightened by use of the tensioning frame, the fiber composite material in the drawing-through gap is loaded with a non-uniform retaining force that is higher in the inlet region on the side of the tensioning frame than in the outlet region.
According to an additional embodiment of the invention, the fiber composite material is drawn over a tool edge, which is arranged on the inlet side of the shaping gap, during the forming phase. This arrangement prevents the formation of disturbing defects in the material and, above all, the formation of folds, which develop in the course of the afterflow of the fiber composite material due to the differences in the local profile lengths and the resulting differences in the tensile stress, induced by said difference in the local profile lengths, at the tensioning frame, from propagating in the shaping gap. As a result, the smoothing effect that is achieved with such an arrangement allows even such preforms that exhibit a very complex contour to be produced with a fiber structure of the highest quality once again in a way that is easy to carry out during the production process.
In an especially preferred embodiment of the invention, the temperature of the tensioning frame is set by heating or cooling to the gelation level of the fiber composite material, so that this material is held at the tensioning frame with practically zero cohesive friction. As a result, it is possible to prevent in an effective way a jerky afterflow of the fiber composite material, said jerky afterflow being induced by the sudden state changes between the cohesive friction at the tensioning frame, on the one hand, and the sliding friction, on the other hand.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.